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Register a new userThe star-formation history of the Universe with the SKA
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Matt J. Jarvis
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Radio wavelengths offer the unique possibility of tracing the total star-formation rate in galaxies, both obscured and unobscured. As such, they may provide the most robust measurement of the star-formation history of the Universe. In this chapter we highlight the constraints that the SKA can place on the evolution of the star-formation history of the Universe, the survey area required to overcome sample variance, the spatial resolution requirements, along with the multi-wavelength ancillary data that will play a major role in maximising the scientific promise of the SKA. The required combination of depth and resolution means that a survey to trace the star formation in the Universe should be carried out with a facility that has a resolution of at least ~0.5arcsec, with high sensitivity at < 1 GHz. We also suggest a strategy that will enable new parameter space to be explored as the SKA expands over the coming decade.
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We present a detailed analysis of the influence of the environment and of the environmental history on quenching star formation in central and satellite galaxies in the local Universe. We take advantage of publicly available galaxy catalogues obtained from applying a galaxy formation model to the Millennium simulation. In addition to halo mass, we consider the local density of galaxies within various fixed scales. Comparing our model predictions to observational data (SDSS), we demonstrate that the models are failing to reproduce the observed density dependence of the quiescent galaxy fraction in several aspects: for most of the stellar mass ranges and densities explored, models cannot reproduce the observed similar behaviour of centrals and satellites, they slightly under-estimate the quiescent fraction of centrals and significantly over-estimate that of satellites. We show that in the models, the density dependence of the quiescent central galaxies is caused by a fraction of backsplash centrals which have been satellites in the past (and were thus suffering from environmental processes). Turning to satellite galaxies, the density dependence of their quiescent fractions reflects a dependence on the time spent orbiting within a parent halo of a particular mass, correlating strongly with halo mass and distance from the halo centre. Comparisons with observational estimates suggest relatively long gas consumption time scales of roughly 5 Gyr in low mass satellite galaxies. The quenching time scales decrease with increasing satellite stellar mass. Overall, a change in modelling both internal processes (star formation and feedback) and environmental processes (e.g. making them dependent on dynamical friction time-scales and preventing the re-accretion of gas onto backsplash galaxies) is required for improving currently used galaxy formation models.
This lecture briefly reviews the major recent advances in radio astronomy made possible by ultra-deep surveys, reaching microJansky flux density levels. A giant step forward in many fields, including the study of the evolution of the cosmic star formation history is expected with the advent of the Square Kilometer Array (SKA).
We explored the impact of the synergy between the Euclid near-infrared photometric surveys and the SKA radio continuum surveys on the studies of the cosmic star formation. The Euclid satellite is expected to perform a Wide and Deep photometric surveys to an infrared limit of H ~ 24 and H ~ 26 respectively and a spectroscopy survey with a flux limit of $sim 3 times 10^{-16}$ erg cm$^{-2}$ s$^{-1}$ in the Halpha line. Combining the H band Euclid selected samples with the ground based ancillary data (fundamental for the SFR estimation) we will be able to detect the star forming galaxies down to SFRs of order of unit to z ~ 2 and down to SFR ~ 10 to z ~ 3, sampling the majority of the star forming galaxies up to z ~3 and beyond and placing definitive constraints on the star formation history of the universe at z<4-5 (is there a peak a z ~2 or a plateau at 1 <z <5 ?) and on the galaxies evolution models. The only tools able to provide a accurate dust-free calculation of their SFR are the SKA continuum surveys. The observational parameters of the Deep Tier SKA1 reference survey (a 0.2- 0.5 arcsec resolution and a 5 sigma detection limit of 1 microJy over 30 deg2 at Band 1/2 ) are the perfect complement of the Euclid survey. We showed, in fact, that with this kind of SKA survey we will be able to determine a dust unbiased SFR for a huge fraction (~85 %) of the Euclid SFG providing strong constraints on the star formation history of the Universe.
We present spatially resolved imaging and integral field spectroscopy data for 450 cool giant stars within 1,pc from Sgr,A*. We use the prominent CO bandheads to derive effective temperatures of individual giants. Additionally we present the deepest spectroscopic observation of the Galactic Center so far, probing the number of B9/A0 main sequence stars ($2.2-2.8,M_odot$) in two deep fields. From spectro-photometry we construct a Hertzsprung-Russell diagram of the red giant population and fit the observed diagram with model populations to derive the star formation history of the nuclear cluster. We find that (1) the average nuclear star-formation rate dropped from an initial maximum $sim10$,Gyrs ago to a deep minimum 1-2,Gyrs ago and increased again during the last few hundred Myrs, and (2) that roughly 80% of the stellar mass formed more than 5,Gyrs ago; (3) mass estimates within $rm Rsim1,pc$ from Sgr,A* favor a dominant star formation mode with a normal Chabrier/Kroupa initial mass function for the majority of the past star formation in the Galactic Center. The bulk stellar mass seems to have formed under conditions significantly different from the young stellar disks, perhaps because at the time of the formation of the nuclear cluster the massive black hole and its sphere of influence was much smaller than today.
The most striking feature of the Cosmic Star Formation History (CSFH) of the Universe is a dramatic drop of the star formation (SF) activity, since z~1. In this work we investigate if the very same process of assembly and growth of structures is one of the major drivers of the observed decline. We study the contribution to the CSFH of galaxies in halos of different masses. This is done by studying the total SFR-halo mass-redshift plane from redshift 0 to redshift z~1.6 in a sample of 57 groups and clusters by using the deepest available mid- and far-infrared surveys conducted with Spitzer MIPS and Herschel PACS and SPIRE. Our results show that low mass groups provide a 60-80% contribution to the CSFH at z~1. Such contribution declines faster than the CSFH in the last 8 billion years to less than 10% at z<0.3, where the overall SF activity is sustained by lower mass halos. More massive systems provide only a marginal contribution (<10%) at any epoch. A simplified abundance matching method shows that the large contribution of low mass groups at z~1 is due to a large fraction (>50%) of very massive, highly star forming Main Sequence galaxies. Below z~1 a quenching process must take place in massive halos to cause the observed faster suppression of their SF activity. Such process must be a slow one though, as most of the models implementing a rapid quenching of the SF activity in accreting satellites significantly underpredicts the observed SF level in massive halos at any redshift. Starvation or the transition from cold to hot accretion would provide a quenching timescale of 1 Gyrs more consistent with the observations. Our results suggest a scenario in which, due to the structure formation process, more and more galaxies experience the group environment and, thus, the associated quenching process. This leads to the progressive suppression of their SF activity shaping the CSFH below z~1.
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